Question: Solve for $x$ and $y$ using substitution. ${-3x-3y = -12}$ ${x = 3y-8}$
Answer: Since $x$ has already been solved for, substitute $3y-8$ for $x$ in the first equation. ${-3}{(3y-8)}{- 3y = -12}$ Simplify and solve for $y$ $-9y+24 - 3y = -12$ $-12y+24 = -12$ $-12y+24{-24} = -12{-24}$ $-12y = -36$ $\dfrac{-12y}{{-12}} = \dfrac{-36}{{-12}}$ ${y = 3}$ Now that you know ${y = 3}$ , plug it back into $\thinspace {x = 3y-8}\thinspace$ to find $x$ ${x = 3}{(3)}{ - 8}$ $x = 9 - 8$ ${x = 1}$ You can also plug ${y = 3}$ into $\thinspace {-3x-3y = -12}\thinspace$ and get the same answer for $x$ : ${-3x - 3}{(3)}{= -12}$ ${x = 1}$